The invention relates generally to outboard and stern drive engines, and more particularly, to apparatus for preventing gases (e.g., exhaust, air) from flowing into a propeller blade.
Through-propeller exhaust type engines include an exhaust casing extending from a power head, and a lower unit secured to the exhaust casing. The lower unit includes a gear case which supports a propeller shaft, and a propeller is engaged to the shaft. The propeller includes an outer hub through which exhaust gases are discharged.
During operation, a region of low pressure is developed rearwardly of the propeller. A thin low pressure boundary layer around the hub can also develop. The low pressure condition rearwardly of the hub has a tendency to join with the low pressure boundary layer, and exhaust gas migrates forwardly along the propeller hub between the blades and along the rear, or low pressure, face of the propeller blades, thereby causing conditions of "cavitation" or "ventilation". Such conditions prevent the propeller blade from biting into the water and result in an efficiency loss. In addition, excessively low pressure in the region rearwardly of the propeller hub results in a drag on the forward movement of the engine through the water.
Known propeller structures for preventing ventilation include diverging flare rings and converging rings at the rear end of the propeller hub. The rings affect the flow of water over the hub and prevent migration of the exhaust gases along the hub. For example, with an aluminum propeller, and after die cast operations, the ring is formed by welding, swaging, or attaching a full-circle ring to the hub.
With such rings, and even during minor underwater impacts, the rings can be damaged and even lost. That is, the rings can be separated from the propeller hub and then sink to the bottom of the river, lake, or ocean. Damage and loss of such rings can result in customer dissatisfaction.
In addition, and with some ring configurations, slots are formed in the ring during fabrication. Formation of the slots in the ring results in high stress areas adjacent the slots, i.e., at the edges of the slots. Such high stress areas, i.e., the edges, are susceptible to cracking and breaking off. Such cracked or broken off edges are not aesthetically acceptable and can result in customer complaints.